Data processing systems comprising at least one host typically require a large amount of data storage. If the data, typically stored as a data volume, is not immediately required by the hosts, for example, if the data volume is infrequently accessed, the storage of the data volume may be on removable rewritable data storage media, such as magnetic tape or optical disk, and the data volumes may be written and or read by means of a data storage drive.
The data storage drive is typically coupled to the host, or processing unit, by means of a peripheral interface in which commands are directed only from the processing unit to the data storage drive, and the data storage drive responds to those commands, performing the commanded functions. No commands can be sent by the data storage drive to the coupled processing unit. Typically, the commands are performed by a device controller.
Data processing systems having multiple hosts require larger amounts of data storage than can be efficiently handled by single data storage drives.
Data storage libraries typically provide efficient access to large quantities of data volumes stored in removable data storage media, the media stored in storage shelves which are accessed by robots under the control of robot controllers. Due to the large amount of stored data, typically, a plurality of hosts make use of the same data storage library, and a plurality of data storage drives are included in the library to allow access by the hosts. A library manager, which may comprise the same processor as the robot controller, typically tracks each data volume and the data storage media on which it is stored, and tracks the storage shelf location of each data storage media.
Herein, a library manager, either with or without the robot controller, is defined as a "controller" for the data storage library, as is the "controller" for a data storage device as discussed above.
An example of a data storage library system for redundantly storing and accessing data volumes stored on removable data storage media in a plurality of data storage libraries is described in the incorporated coassigned K. F. Day III et al application. The library controller of each library provides an updatable synchronization token directly associated with each data volume. A plurality of directors are provided, each separate from and coupled to the hosts and each separate from and coupled to each data storage library. Each director responds to separate, partitioned data storage drive addresses addressed by the hosts. The responding director supplies each data volume supplied from a host to all of the data storage libraries, and updates each synchronization token directly associated with the supplied data volume. Thus, the directors store duplicate copies of the data volume in the data storage libraries without involvement by the host. In most data processing applications, it is critical to access the most current data. Hence, the currency of the data volumes are each tracked by means of the directly associated synchronization token, and the synchronization token is not tracked by the host.
In the instance where one copy of the data volume is down level and the corresponding synchronization token has not been updated, the responding director compares the tokens from all the libraries to identify the down level data volume, so that the erroneous down level information is not transferred to hosts. The responding director may then update the most current data volume and the corresponding synchronization token and the previously down level volumes are also updated.
If, as a data volume is being updated at one of the libraries, another of the libraries is or goes temporarily out of service, or out of communication with the directors, the data volume at that library cannot be updated. Thus, when the copy of the data volume on the operating library is updated, that volume is exposed, in that one copy will be more current than another. Later access to the data, and conducting the comparison of the tokens, will note the lack of consistency of the data, but if the library having the most current copy is then temporarily out of service, or out of communication with the directors, there will be no ability to note that the accessed data is down level so long as the library is out of service, etc. Further, the down level data may be further updated without knowledge that it is down level, and its token updated, so that the updated data may be lost forever.